The present invention relates generally to long-haul, fiber optic transmission systems. More particularly, the present invention relates to increasing the capacity of undersea cable systems.
An undersea network of fiber-optic telecommunication cables links continents, nations and lands together. Communication traffic on these submarine cables has increased dramatically in recent years and is expected to continue to grow as a result of many factors, including, for example, the globalization of world economies and businesses, the increased demand for international communications capabilities, and the development of multimedia applications and revolutionary resources such as the Internet. These factors necessitate increasing capacity not only by adding cables, but also by optimizing the operation of existing submarine cables. The term xe2x80x9ccapacityxe2x80x9d as used herein refers to the total bit rate of the cable system which represents the sum of the bit rates of each channel on all the fibers within a cable.
Approaches for increasing capacity on existing cables have traditionally been aimed at optimizing bandwidth utilization. Long-haul optical fiber transmission paths, such as those employed by undersea cables, typically extend over 5000 km and operate in the C-band. The C-band spans from about 1525 to about 1565 nm and is optimal for low-absorption losses. Within the C-band, capacity may be increased by increasing the number of channels and the bit rate of the channels. Most long-haul cable systems increase the number of channels through wavelength division multiplexing (WDM). The ultimate capacity of a WDM cable system depends on how closely channels can be packed. Minimum channel spacing is limited, however, by interchannel crosstalk and the degradation of the signal to noise ratio (SNR). It has been found that, to maintain sufficient signal integrity, channel spacing in GHz should exceed, e.g., four times the bit rate in Gb/s. Therefore, for a given bandwidth, the number of channels and bit rate are interrelated and limit the capacity of a cable.
Furthermore, in practice, other factors restrict the use of the entire low-loss bandwidth window of 120 nm near 1550 nm. For example, the number of channels is limited to the bandwidth over which the amplifiers can provide nearly-uniform gain. Other factors that limit the number of channels include nonlinear effects and the tunability of laser transmitters. Therefore, increasing channel bit rate and the number of multiplexed channels is limited by SNR minimums and the current state of the technology.
One approach to increase useable bandwidth involves using the L-band in addition to the C-band. The L-band refers to a bandwidth of about 1570 to about 1610 nm. Combining the L-band with the C-band expands the useable bandwidth from about 1525-1565 (40 nm) to about 1525-1610 nm (80 nm). Use of the L-band has been limited in the past, however, due to several factors, one of the more significant being Raman effects.
In multiple channels systems, the fiber acts as a Raman amplifier such that longer wavelength channels are amplified by shorter wavelength channels when the wavelength difference is within the bandwidth of the Raman gain. The Raman gain of silica fibers is so broad that the amplification can occur for channels spaced as far apart as 200 nm, although the peak amplification occurs between about 110 and about 120 nm from the pump wavelength. The shortest wavelength becomes the most depleted as it can pump many long-wavelength channels simultaneously. It is interesting to note that amplification only occurs when 1 bits are present in both channels simultaneously. This signal-dependent amplification leads to enhanced power fluctuations which add to receiver noise and degrade receiver performance.
The Raman effects between the C- and L-bands are particular problematic compared to the effects within just the C-band, especially in long-haul systems. Given the conventional operating bandwidth within the C-band of less than 40 nm, the Raman effects between channels tend to be insignificant since the peak Raman Stokes shift is about 110 to about 120 nm from the pump wavelength. However, when the C- and L-bands are combined and form a bandwidth of close to 80 nm, the wavelength difference between the shortest and longest channels is quite near the peak Raman Stokes shift.
Aside from Raman effects, combining the C and L-band also is problematic from the standpoint of isolation. More specifically, to isolate bands based on wavelength, a minimum bandgap between them is necessary to maintain the integrity of the channels at the interface of the C- and L-bands. This bandgap tends to be relatively large compared to the spacing between channels and consumes valuable bandwidth thereby reducing capacity.
Therefore, a need exists for increasing capacity on new and existing lines using known technologies while maintaining the integrity of the signals. The present invention fulfills this need among others.
The present invention provides an approach for increasing capacity on long-haul cable systems that overcomes the aforementioned problems by employing counter-propagating band signals, preferably, counter-propagating C-band and L-band signals. By using counter-propagating band signals, the Raman effects associated with co-propagating bands are significantly reduced. Applicants suspect that the Raman effects are greatly reduced for counter-propagating signals due to xe2x80x9cwalk offxe2x80x9d and/or xe2x80x9cpower distributionxe2x80x9d effects (described in detail below), although the scope of the present invention is not, in any way, tied to a particular theory. In addition to the benefits of reducing Raman effects, counter-propagating band signals also are more readily isolated. It is more efficient to isolate bands based on their direction of propagation, rather than on their wavelength differences, since a relatively-large bandgap between the bands is not needed to improve the isolation.
In effecting counter-propagating C- and L-band signals on long-haul cable systems, such as undersea cable systems, it has been found that a reduction in backward Rayleigh scattering (BRS) is required. BRS results from random localized variations of the molecular positions in glass that create random inhomogeneities of the reflective index that act as tiny scatters centers. Although BRS is not a problem with counter-propagating band signals over relatively-short transmission paths, such as terrestrial lines of 100-300 km (see, for example, Suzuki et al. Bidirectional Ten-Channel 2.5Gbitls WDM Transmission over 250 km Utilizing 76 nm (1531-1607 nm) Gain-band Bidirectional Erbium-doped Fiber Amplifiers, Optical Amplifiers and Their Applications (IEEE/Lasers and Electro-Optics Society, July 1997)), BRS tends to accumulate on long-hauls systems having many cascaded amplifiers. The BRS causes problems in amplification, especially for the L-band which is particularly susceptible to C-band power induced from the BRS of the C-band signals (see, for example, Massicott et al. Low Noise Operation of EY+Doped Silica Fibre Amplifier Around 1-6 xcexcm (August 1992)). More specifically, as the BRS level rises through accumulation over many amplifiers, it can induce gain and output power fluctuations.
The applicants not only have identified the problems with BRS associated with counter-propagating band signals on long-haul optical fibers, but also have developed a solution. More specifically, it has been found that the cascading effects of BRS can be reduced significantly by isolating and filtering each bandwidth during optical amplification.
The applicants also have developed an approach for isolating and filtering the band signals without incurring high insertion losses and noise figure (NF). More specifically, a novel configuration of bandsplitters is set forth which exploits certain characteristics of bandsplitters. Specifically, a bandsplitter""s reflective port has lower insertion loss and NF than its transmission port, while its transmission port has better isolation than its reflective port. Accordingly, in a preferred embodiment, the reflective ports of two or more bandsplitters are used as the xe2x80x9cinputxe2x80x9d into an amplifier system such that each bandsplitter reflects signals of a particular band onto a particular transmission path for amplification. Because the reflective ports are used as the input, the insertion loss and NF are minimized. The transmission ports of the bandsplitters are used as the xe2x80x9coutputxe2x80x9d from the amplifier system such that each bandsplitter transmits the amplified signals of a particular band onto the optical fiber having counter-propagating band signals. Because the transmission port is used as the output, a high level of isolation is achieved before the band signals are transmitted onto the optical fiber. Therefore, the bandsplitter configuration splits and filters band signals while introducing minimal insertion losses and NF and achieving high isolation.
Accordingly, one aspect of the present invention is a method of operating a cable by using counter-propagating bandwidths. In a preferred embodiment, the method comprises the steps of: (a) effecting counter-propagating first and second signals of different bands on a common long-haul optical fiber having cascaded optical amplifiers; and (b) band amplifying the first and second signals while reducing BRS on the optical fiber . As used herein, the term xe2x80x9ceffecting counter-propagating signalsxe2x80x9d refers broadly to taking any action or participating in any way that results in signals counter-propagating on a long-haul optical fiber and includes, for example, operating a cable system or any portion thereof to transmit and/or receive signals in opposite directions on the same optical fiber, and operating a cable system to transmit signals on an optical fiber in one direction during one time period (e.g. daytime) and in a reverse direction during another time period (e.g., nighttime).
Another aspect of the present invention is a method for amplifying counter-propagating band signals over a long distance between cable stations. In a preferred embodiment, the method comprises the steps of (a) splitting counter-propagating signals into two or more bands; (b) reducing BRS within each band; (c) amplifying each band; and (d) combining the bands after steps (b) and (c) on an optical fiber to effect counter-propagating signals on the optical fiber. Preferably, a configuration of bandsplitters is used to split and combine the bands.
Yet anther aspect of the invention is an amplification system for amplifying counter-propagating signals on a long-haul cable system. In a preferred embodiment, the amplifier system comprises: (a) band splitting/combining optics configured for splitting counter-propagating signals into two or more bands onto separate transmission paths, and for combining the bands on a common optical fiber; (b) BRS-reduction optics for reducing BRS on each separate transmission path; and (c) at least one gain-band amplifier disposed on each separate transmission path for band amplifying the particular band thereon. Preferably, the band splitting/combining optics comprises a configuration of bandsplitters.
Still another aspect of the present invention is a long-haul cable system that supports counter-propagating signals. In a preferred embodiment, the system comprises: (a) a long-haul optical fiber comprising at least one optical fiber; (b) two or more cable stations connected to the optical fiber; and (c) a plurality of amplifier systems as described above disposed along the optical fiber.